We present a comprehensive study of the nucleation kinetic of Cu on Ni͑100͒ using variable-temperature scanning tunneling microscopy. The analysis of the saturation island density as a function of substrate temperature and deposition rate reveals that the smallest stable island abruptly changes from a dimer to a tetramer. From the Arrhenius plot, the migration barrier E m ϭ͑0.35Ϯ0.02͒ eV, as well as the dimer bond energy E b ϭ͑0.46Ϯ0.19͒ eV, has been deduced. For low ratios between the migration constant D and flux R (D/RϽ10 4 ), nucleation and island growth take place not only during, but also after deposition. In this postnucleation regime, the final island density and island size distribution are no more determined by the competition between flux and monomer migration, but solely by the monomer concentration present immediately after deposition. Therefore, the island density becomes independent of substrate temperature and flux, and the scaled island size distribution closely resembles that of statistic growth ͑adatom smallest stable island͒. The experimental results are compared with simulations using rate equations. ͓S0163-1829͑96͒10948-6͔
A newly established combination of a femtosecond laser with a low temperature scanning tunneling microscope is described, which facilitates one to analyze femtochemistry on metal surfaces in real space. The combined instrument enables focusing the laser to some tens of micrometers and guiding it reproducibly into the tunneling gap with the aid of in situ movable mirrors. Furthermore, a method to determine the focus size on the sample is presented. The focus size is used to calculate the electron and phonon temperatures at the surface. Despite the additional noise introduced by laser operation the vertical resolution of the microscope lies below 1 pm. The potential of the instrument is demonstrated on para-chloronitrobenzene clusters adsorbed on Au(111). Single chloronitrobenzene molecules diffuse upon femtosecond laser irradiation; some smaller clusters rotate by multiples of 30 degrees ; clusters of less compact form rearrange to close-packed clusters.
A novel mechanism for ramified island growth in the initial stages of metal heteroepitaxy is reported. Scanning tunneling microscopy measurements reveal that copper islands on Ni(100), as they grow in size, undergo a shape transition. Below a critical size of ഠ480 atoms, compact islands form, while above this size they develop a ramified shape. This effect is not of kinetic origin and has been observed in an extended range of growth temperature (250 -370 K) and deposition flux ͑1025 10 22 monolayer͞s͒. The shape transition is ascribed to the island size dependent strain relaxation. [S0031-9007(98)
We report a novel mechanism, internal (111) faceting, of strain relief at heterointerfaces with square symmetry. The mechanism has been revealed for thin Cu films on Ni(100) by scanning tunneling microscopy. In the first monolayer monatomic chains of Cu atoms are shifting laterally by 1͞ p 8 lattice constant along ͗110͘ and thereby protrude from the surface layer. With each Cu layer added, the protrusion stripes grow in width by one atom, forming internal ͕111͖ facets in the Cu film. This picture is in marked contrast to the widely accepted continuum theory of epitaxial growth, which predicts a pseudomorphic film growth up to a critical thickness of 8 monolayers.
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